The invention described herein relates generally to carbon foams, and especially to processes for preparing low density microcellular carbon, and catalytically impregnated carbon, foams.
There are many, both presently existing and potential, beneficial uses for carbon foams. For example, carbon foams have been used as parts for inertial confinement fusion targets, as absorbers of toxic and hazardous gases, and as structural parts requiring unique properties related to X-ray opacity. Very possibly, in applications where large and highly reactive surface areas of catalysts such as platinum, palladium and nickel must be exposed to unsaturated hydrocarbon gases in their catalytic decomposition, it appears that it would be highly beneficial to provide low density, microcellular foams comprised of catalytically impregnated carbon. In these and similar applications, it would clearly be very advantageous to make carbon foams having both a cell size and a density that were, at the same time, independently controllable. It would be especially advantageous to fabricate carbon foams simultaneously having a low density and a very small cell size. It would be beneficial if the small cell size were very uniform and could be tailored to be within the 5 to 30 micron range. Additionally, it would be highly desirable if these tailored carbon foams were free of impurity and structurally stable so that they could be easily fabricated into parts of various size and shape. Unfortunately, the presently known methods for preparing carbon foams are inadequate for producing these tailored foams.
One prior art method for preparing carbon foam is described by Benton et al in Carbon 10, pages 185 to 190 (1972). In this method, hollow phenolic or carbon microspheres are mixed with a binder material consisting of liquid furfuryl alcohol, maleic anhydride, powdered pitch, and acetone. The moist mixture is cured under conditions of elevated temperature and pressure. Then the cured mixture is carbonized at high temperature in an argon-purged furnance. This carbonizing, or coking operation produces a significant shrinkage of the resultant carbon foam, which possesses a high compressive strength and a relatively low density. However, the cell size of the carbon foam made by this method tends to be relatively large, and cell size and density cannot be simultaneously and independently controlled to provide carbon foams of low density and small cell size.
Processes for producing reticulated, or weblike, polymeric foams by removing the cell membranes from conventional polymeric foams are described by Geen in the "Encyclopedia of Polymer Science and Technology", Volume 12, pages 102 to 104, Interscience Publishers (1970). Polyester-derived polyurethane foams may be reticulated by alkaline hydrolysis, which preferentially etches away the foam membranes, leaving an open skeletal structure. In another method, called explosion reticulation, the air within a foam is removed and replaced with an explodable gas mixture. Ignition of the mixture results in a controlled explosion which removes the thinner membranes. Reticulated carbon foams can be produced by the pyrolysis of polyurethane foams that have been reticulated by either of these methods. Unfortunately, the resulting reticulated carbon foams cannot be fabricated to simultaneously meet the aforementioned desirable cell size and density specifications.
Thus, the problem remains of readily preparing machinable and structurally stable, tailored low density and microcellular carbon, and catalytically impregnated carbon, foams.